Powertrain systems on vehicles provide power to one or more of the vehicle wheels in response to a driver accelerator request such as is received typically in the form of an accelerator pedal input. Vehicle launch assist (VLA) and traction control (TC) functions may be used by the powertrain control system to limit the power (torque) applied in response to the driver input so as to reduce wheel slip between the driven wheel(s) and ground surface over which the vehicle is traveling. A powertrain control system may use a VLA function for low speed operation and sometimes for particular surface terrain conditions, such as when driving on sand. The TC function may then be used for more typical driving speeds and conditions.
In a known TC function, the driver accelerator input is used to generate a filtered torque demand representing the maximum torque to be supplied to the vehicle wheel(s). At lower speeds (e.g., under 5 km/h) and in at least some terrain conditions such as sand, the VLA function is used in lieu of the TC function to limit the maximum torque applied to the wheel(s) to something less than that provided by the TC function under those same conditions. Then, as vehicle speed increases (e.g., past 5 km/h), the VLA function is exited and the powertrain control system switches to TC function. The torque limit used then switches from the maximum allowed by the VLA function to typically a much large value that which would have been available at that point in time from the TC function had it been utilized. This jump is done by factoring out the VLA function influence as it switches to the TC function, which may involve an high rate of torque increase that is undesirable.
In particular, handing over torque control from the VLA function to TC function in this way may result in increasing torque at a rate that exceeds the TC function torque profile for those same conditions. And, where driver torque demand (the driver accelerator request) has gone away by the time the VLA function is being exited, this approach may result in an increasing torque occurring during this transition period even though the drive torque demand has ended.